Mercuric reductase is a homodimeric protein with two interfacial active sites per dimer. Extensive evidence, obtained with enzyme from a Pseudomonas transposon Tn501, indicates that the active site environments are asymmetric when pyridine nucleotide substrates are bound at both sites, but symmetric in the absence of ligands. Since the complexed enzyme is catalytically relevant, we proposed a role for the asymmetry in the cataytic mechanism. To elucidate the mechanistic role, we examined a number of ligands to determine which portions of ligands may be involved in induction of asymmetry. In addition, we evaluated the oxidative half- reaction of the enzyme with a variety of Hg(II) compounds, which indicates that the intersubunit interaction may primarily result from the nature of the Hg(II) substrate as it is presented to the enzyme in vivo. With simpler Hg(II) salts, the need for intersubunit communication may be abolished. With the new information, we are using MidasPlus to identify specific pathways for communication between the active sites and to evaluate points of access for the different Hg(II) compounds to the active site. The Computer Graphics Laboratory provides a valuable resource for our efforts to understand the complex.